Monolithic integration of multiple compound semiconductor FET devices

ABSTRACT

Various aspects of the technology provide a dual semiconductor power and/or switching FET device to replace two or more discrete FET devices. Portions of the current may be distributed in parallel to sections of the source and drain fingers to maintain a low current density and reduce the size while increasing the overall current handling capabilities of the dual FET. Application of the gate signal to both ends of gate fingers, for example, using a serpentine arrangement of the gate fingers and gate pads, simplifies layout of the dual FET device. A single integral ohmic metal finger including both source functions and drain functions reduces conductors and contacts for connecting the two devices at a source-drain node. Heat developed in the source, drain, and gate fingers may be conducted through the vias to the electrodes and out of the device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the priority benefit ofU.S. patent application Ser. No. 13/270,145, filed Oct. 10, 2011 nowU.S. Pat. No. 8,274,121, and titled, “COMPOUND FIELD EFFECT TRANSISTORWITH MULTI-FEED GATE AND SERPENTINE INTERCONNECT,” which is continuationof and claims the priority benefit of U.S. patent application Ser. No.13/205,433, filed Aug. 8, 2011, and titled “LOW INTERCONNECT RESISTANCEINTEGRATED SWITCHES,” which in turn claims the priority benefit of U.S.provisional application No. 61/372,513, filed Aug. 11, 2010, and titled“Field Effect Transistor and Method of Making Same.” This application isalso related to U.S. patent application Ser. No. 13/364,258, filed Feb.1, 2012, and titled “SELF CLAMPING FET DEVICES IN CIRCUITS USINGTRANSIENT SOURCES.” The above referenced applications are herebyincorporated by reference in their entirety.

1. Technical Field

The present invention relates to semiconductors devices, and moreparticularly to compound semiconductor Field Effect Transistor switchesand power FETs.

2. Background

A common type of Field Effect Transistors (FET) is aMetal-Oxide-Semiconductor Field Effect Transistor (MOSFET), which may befabricated using silicon. A FET may also be fabricated using germaniumor a compound semiconductor such as gallium arsenide (GaAs) or galliumnitride (GaN). FET devices fabricated from compound semiconductors suchas GaAs make very good switches and signal amplification devices for rfand microwave applications. Among these devices are switches andlarge-signal (or power) amplifier circuits. Some advantages of compoundsemiconductor FET switches over silicon MOSFET switches include highblocking (off-state) resistance, low on-state resistance (RDs (on)),fast switching speed, high current density, low temperature coefficient,high junction temperature and, for GaN devices, high breakdown voltage.Unfortunately, compound semiconductor FET switches and power FETs arealso more expensive to manufacture than silicon MOSFETs due to thelarger size of the FETs necessary to handle power, smaller wafers andhigher fabrication expenses. Merely decreasing the size of compoundsemiconductors by scaling down the device may not decrease costs.

SUMMARY

Device cost of a compound semiconductor switching or power FET is drivenby two factors, namely size and yield. The present invention addressesboth. Reducing the size while maintaining current handling capabilitiesis accomplished by distributing portions of the current handled by thedevice in parallel to sections of the source and drain fingers tomaintain a low current density and eliminate the need for outboardbonding pads. Increasing yield is accomplished by applying the gatesignal to both ends of the gate fingers, which eliminates a major sourceof device failure, i.e., a break in any single one of the many gatefingers. The current to be handled by the FET may be divided among a setof electrodes arrayed along the width of the source or drain fingers.The electrodes may be oriented to cross the fingers along the length ofthe array of source and drain fingers. The portion of the currentdistributed to each source electrode may be coupled to a section of eachsource finger crossed by the source electrode. Similarly, the portion ofthe current distributed to each drain electrode may be applied to asection of each drain finger crossed by the drain electrode. The currentmay be conducted from the source and drain electrodes to the source anddrain fingers, respectively, through vias disposed along the surface ofthe fingers. Heat developed in the source, drain, and gate fingers maybe conducted through the vias to the electrodes and out of the device.Moreover, the overall size of a circuit including two discrete FETdevices, such as a control FET and a sync FET may be reduced in size andcost by integrating the control FET and the sync FET as a singlecompound semiconductor device and fabricating the source of the controlFET and the drain of the sync FET as a single set of continuous integralohmic metal fingers. Serpentine gate structure may accommodate a nodebetween contiguous source and drain fingers (e.g. a source finger of acontrol FET and a drain finger of a sync FET).

Various aspects of a dual Field Effect Transistor (FET) device include acompound semiconductor layer and a control FET fabricated on thecompound semiconductor layer. The control FET includes an ohmic metalcontrol source finger and an ohmic metal control drain finger disposedon a surface of the compound semiconductor layer, a control gate fingerbetween the control source finger and the control drain finger, and afirst and second control gate pad at opposite ends of the control gatefinger and in electrical contact with the control gate finger. The dualFET device further includes a sync FET fabricated on the compoundsemiconductor layer with the control FET as a monolithic device. Thesync FET includes an ohmic metal sync source finger and an ohmic metalsync drain finger disposed on the surface of the compound semiconductorlayer, a sync gate finger between the sync source finger and the syncdrain finger, and a first and second sync gate pad at opposite ends ofthe sync gate finger and in electrical contact with the sync gatefinger, the first control gate pad and the first sync gate pad disposedbetween the control drain finger and the sync source finger. The dualFET device also includes a node between the control source finger andthe sync drain finger. The control source finger and the sync drainfinger form a continuous ohmic metal finger including the node.

Various aspects of a method for switching current using a control FETand a sync FET include partitioning a current into a plurality ofcurrent segments and distributing the plurality of current segments tosections of a control drain element of a control FET, the currentsegments distributed through a plurality of vias distributed along asurface of the control drain current element. The method furtherincludes coupling a control gate signal to a first and second end of acontrol gate finger disposed between the control drain element and acontrol source element of the control FET, switching the plurality ofcurrent segments from the control drain element to the control sourceelement using the control gate, and conducting the switched currentsegments along a continuous ohmic metal from the control source elementof the control FET to a sync drain element of the sync FET. The methodalso includes extracting the plurality of current segments from sectionsalong the sync drain element through a plurality of vias distributedalong a surface of the sync drain element.

In various embodiments, the Field Effect Transistor device comprises acompound semiconductor layer, a plurality of control source fingers andsync source fingers disposed on a surface of the semiconductor layer,and a plurality control drain fingers and sync drain disposed on thesurface of the semiconductor layer, the control drain fingersalternating with the control source fingers, and the sync drain fingersalternating with the sync source fingers, each of the plurality ofcontrol source fingers integrally connected to and forming a continuousohmic metal with a corresponding sync drain finger. A plurality ofcontrol gates may be disposed between adjacent control source fingersand control drain fingers, and a plurality of sync gates may be disposedbetween adjacent sync source fingers and sync drain fingers. A pluralityof first control gate pads may be disposed opposite the control gatefingers from a plurality of second control gate pads, each of the firstand second control gate pads configured to couple a control gate signalto two of the control gate fingers. Each of the plurality of secondcontrol gate pads may be disposed between one of the plurality ofcontrol drain fingers and a respective sync source finger. A pluralityof first sync gate pads may be disposed opposite the sync gate fingersfrom a plurality of second sync gate pads, each of the first and secondsync gate pads configured to couple a sync gate signal to two of thesync gate fingers. Each of the plurality of second sync gate pads may bedisposed between one of the plurality of control drain fingers and arespective sync source finger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of a conventional layout for a prior art largeperiphery power FET.

FIG. 2 is a cross section view of the FET of FIG. 1 along line a-a.

FIG. 3 is a plan view illustrating a typical unit cell of section ofactive device area of the FET of FIG. 1.

FIG. 4 illustrates a unit cell of a section of a reduced size for acompound semiconductor FET, in accordance with embodiments of thetechnology.

FIG. 5A is a perspective cutaway view of a block diagram for a FETdevice according to various aspects of the technology.

FIG. 5B is a block diagram of a side elevation illustrating layers ofthe FET device 500 of FIG. 5A.

FIG. 6 is a top plan view of the cut-away of the FET device of FIG. 5A.

FIG. 7 is an exploded view without cutaway of the FET device of FIG. 5A.

FIG. 8 illustrates details of an arrangement of the ohmic layer of FIG.5A.

FIG. 9 illustrates details of the topology of the first metal layer ofFIG. 5A.

FIG. 10 illustrates details of a second metal layer of FIG. 5A.

FIG. 11 illustrates an alternative embodiment of the layout illustratedin FIG. 8, in accordance with various aspects of the invention.

FIG. 12 illustrates an alternative embodiment of the layout illustratedin FIG. 8, in accordance with various aspects of the invention.

FIG. 13 illustrates an alternative embodiment of the layout illustratedin FIG. 8, in accordance with various aspects of the invention.

FIG. 14 illustrates a typical circuit diagram for a buck converter.

FIG. 15 illustrates an elevation view of a prior art implementation ofthe buck converter circuit of FIG. 14.

FIG. 16 illustrates details of an alternate embodiment of a layout of anohmic and gate metal layers of FIG. 5A for implementing the circuit ofFIG. 14 in accordance with embodiments of the invention.

FIG. 17 is a breakaway view illustrating details of a topology of afirst metal layer in relation to the ohmic layer of FIG. 16.

FIG. 18 illustrates a top plan view of the first metal layer of FIG. 17.

FIG. 19 illustrates a top plan view of a second metal layer.

FIG. 20 illustrates the second metal layer in relation to the firstmetal layer.

FIG. 21 illustrates a top plan view of an alternative embodiment of thesecond metal layer.

FIG. 22 is a block diagram illustrating layers of a side elevation of aFET device of FIGS. 16-21.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a conventional layout for a prior artlarge periphery power FET 100. FIG. 2 is a cross section view of the FET100 of FIG. 1 along line a-a. The FET 100 includes source fingers 102,drain fingers 104 and gate fingers 106. The source fingers 102 and drainfingers 104 may be ohmic metal fabricated on an N-type or P-typesemiconductor 120, (or compound semiconductor epitaxial layer) which isdisposed on a semi-insulating substrate (not illustrated) such assilicon or GaAs. The term ohmic metal is used to refer specifically tosource metal, which is metal used in source fingers, and to drain metal,which is metal used in drain fingers. Source and drain metal may be inlow resistance contact with the compound semiconductor epitaxial layer.This may be achieved by depositing a specific set of materials (e.g.,Au, Ge, and/or Ni) then heating the wafer so that the metals alloy (ordiffuse) into the epitaxial layer creating the low resistanceconnections. In some embodiments, gate metal, which is used in gatefingers, comprises a set of deposited metals (e.g., Ti, Pt, Au, and/orAl). Gate metal forms a Schottky contact with the surface of theepitaxial layer, creating the Schottky diode structure in the region ofepitaxial layer that comprises the gate region.

In operation, current flows between the source fingers 102 and the drainfingers 104. The amount of current flowing is controlled by a voltageapplied to the gate fingers 106. The FET 100 further includes a drainpad 114, source pad 108, and a gate pad 116. An air bridge 110 providesinterconnections between the source fingers 102, through contacts 112 tothe source fingers 102 and to the source pads 108. The contacts 112 areshown in dotted line to indicate that they are between the air bridge110 and the source fingers 102 or source pad 108. A length of the sourcefingers 102, drain fingers 104, and gate fingers is measured in thehorizontal axis as illustrated FIG. 1 and is generally the shortdimension. A width of the source fingers 102, drain fingers 104, andgate fingers is measured in the vertical axis as illustrated FIG. 1 andis generally the long dimension. A “gate periphery” may be a measurementof an active area of a FET (or an active region of the FET underconsideration). The gate periphery is generally a number of gate fingersdistributed along the length of the device (the horizontal axis inFIG. 1) times the width of the gate fingers (in the long axis orvertical axis of FIG. 1). For example, a FET (or a region of a FET) thathas 100 gate fingers, each 1 mm in width, has a gate periphery of 100mm.

A device such as the FET 100 has a large footprint requiring a greatdeal of expensive wafer surface. This large die size is generally drivenby a number of factors: The first factor is a requirement for manysource and drain fingers in the active area of the device to support alarge gate periphery. The second factor is a requirement that the drainfingers 104 are long enough (in the horizontal direction in FIG. 1) toconduct current without failing due to generating too much heat. Thethird factor is that the length of the source fingers 102 is driven bythe process technology used to form the air bridge, thus, source fingers102 must be long enough (in the horizontal direction in FIG. 1) toaccommodate the contacts 112 to the air bridge 110. The fourth factor isa requirement for large outboard pads, e.g., the drain pad 114, thesource pads 108, and the gate pad 116.

FIG. 3 is a plan view illustrating a typical unit cell 310 of section300 of active device area of the FET 100 of FIG. 1. The source fingers102 and drain fingers 104 are 30 microns each in length and the channels118 in which the gate fingers 106 are positioned are 5 microns inlength. Thus, an example unit cell 310 of the device (represented by adotted line rectangle) having a 70 micron length×100 micron width (7,000sq. microns) would encompass two gates, each 100 microns wide, or 200microns of gate periphery or “active” device area.

GaAs devices typically have a specific resistivity of around one ohm-mm,so in order to achieve on-state resistances in the milliohm range, verylarge FETs, with gate peripheries on the order of hundreds ofmillimeters, are required. This large gate periphery is the major yielddriver (and major cost factor) in the manufacturing of such devices.Thus, a device as illustrated in FIG. 3 might require about 7,000,000square microns (7 mm²) of active device area, in addition to peripheralpads to achieve 200 mm of gate periphery.

FIG. 4 illustrates a unit cell 410 of a section 400 of a reduced sizefor a compound semiconductor FET, in accordance with embodiments of thetechnology. The size of a compound semiconductor FET device may bereduced by reducing widths of the source fingers 402 and drain fingers404 as illustrated in FIG. 4. For example, a source finger 402 and adrain finger 404, each having a length of about 7 microns may produceabout three times the gate periphery in about the same size unit cell410 (about 72×100 microns as illustrated in FIG. 4). Note that it maynot be practical to shrink the length of the channel 418 in proportionto the unit cell because of various device performance restrictions suchas breakdown voltage. Note also that because of the symmetrical natureof the ohmic metal structure of a FET, source and drain fingers may beinterchangeable. The embodiment illustrated in FIG. 4 may achieve 600 mmof gate periphery in the unit cell 410 which is about same size as theunit cell 310.

As it turns out, there are a number of barriers to simply scaling a FETdevice such as illustrated in the section 300 of FIG. 3 down to a FETdevice as illustrated in the section 400 of FIG. 4. As discussed above,there is a limit to how much the length the drain fingers 104 can bereduced and still carry adequate current from the pad 114 through theentire width of the drain fingers 104. As the cross section of sourcefingers 102 and the drain fingers 104 decreases metal migration occursin the direction of the current, further decreasing the cross section.Further, as the cross section of the fingers decrease the resistance inthe fingers increases. A practical limit for reduction of the length ofthe source fingers and the drain fingers 104 is about 30 microns.

Moreover, there are additional limits to simply scaling down variouscomponent parts of a FET device. For example, scaling down the length ofthe gate fingers 106 (or 406) can result in an increase in defect ratesdue to breaks in the gate fingers 106. This in turn can reduce yield. Itturns out that as the length of the gate fingers is reduced, theprobability of a break in the gate fingers 106 increases. For example, areduction in length of the gate fingers 106 to about 0.25-0.5 micronscould substantially decrease a yield for a FET device having a 1 metergate, to less than 40%. While, reducing the length of the gate fingers106 may have limited bearing on the total size of a FET, there may beother reasons for wishing to decrease the length.

Another limit to scaling down a FET device turns out to be a limitationon spacing between gate fingers 106 (gate pitch) imposed by temperaturecontrol. Most of the heat is generated in the FET device 100 and isgenerated under the gate fingers 106 and is conducted out of the devicethrough the semiconductor 120 and the substrate. A compoundsemiconductor such as GaAs is a rather poor thermal conductor. The heattends to propagate in a spreading action away from gate fingers 106through the semiconductor 120 and substrate at about 45 degrees, asillustrated in FIG. 2. The heat spreading action tends to increase thearea through which heat is removed from the gate region and improvesefficiency for removing heat from the gate region. However, as the FETdevice is scaled down, heat propagating at 45 degrees from adjacent gatefingers 106 interferes with the spreading action, and efficiency of theconduction of heat through the semiconductor 120 and substratedecreases. Yet another barrier is that the air bridge 110 illustrated inFIG. 1 is precluded because of the narrow source fingers 402.

FIG. 5A is a perspective cutaway view of a block diagram for a FETdevice 500 according to various aspects of the technology. FIG. 5B is ablock diagram of a side elevation illustrating layers of the FET device500 of FIG. 5A. FIG. 6 is a top plan view of the cut-away of the FETdevice 500 of FIG. 5A. FIG. 7 is an exploded view without cutaway of theFET device 500 of FIG. 5A. The arrangement of the components of the FETdevice 500 may provide a solution to a number of problems in scaling acompound semiconductor FET down to a smaller size. The FET device 500includes a semiconductor layer 550 and an ohmic layer 510 disposed onthe semiconductor layer 550. The semiconductor layer 550 may be a P-typeor N-type semiconductor and may be fabricated using compoundsemiconductors such as GaAs and GaN. The semiconductor layer may bedisposed on an insulating or semi-insulating substrate 560. Examples ofa substrate layer include GaAs, Si-carbide, Si, and sapphire. Duringfabrication the substrate layer may be ground down to 50-100 microns.The FET device 500 further includes a first dielectric layer 528disposed on the ohmic layer 510, and a first metal layer 520 disposed onthe first dielectric layer 528. The FET device 500 further includes asecond dielectric layer 538 disposed on the first metal layer 520 and asecond metal layer 540 disposed on the second dielectric layer. Thefirst dielectric layer 528 may cover a substantial portion or the entiresurface of the FET device 500, including ohmic metal, gate metal and theexposed surface of the epitaxial layer between the gate metal and theohmic metal. The first dielectric layer 528 may seal the covered surfaceand/or any embedded structures (e.g., vias) from the outsideenvironment, protecting against accidental damage and exposure tomicroscopic particles. This, in turn, may eliminate the need for anexternal package which is often required to achieve such a level ofenvironmental protection. Similarly, the second dielectric layer 538 maycover, seal, and/or protect the second metal layer 540. The firstdielectric layer 528 and/or the second dielectric layer 538 mayhermetically seal the device surface. In various embodiments, the firstand second dielectric material includes silicon dioxide, silicon oxide,fluorine-doped silicon dioxide, carbon-doped silicon dioxide, poroussilicon dioxide, porous carbon-doped silicon dioxide, and/or the like.The first dielectric layer 528 and second dielectric layer 538 areomitted in FIG. 5A for clarity and illustrated in FIG. 5B in blockdiagram form.

FIG. 8 illustrates details of an arrangement of the ohmic layer 510 ofFIG. 5A. The ohmic layer 510 includes source fingers 502 alternatingwith drain fingers 504. A gate finger 506 is disposed in a gate channel518 between each adjacent source finger 502 and drain finger 504. Ohmicmetals provide low resistance contact to the semiconductor layer 550.The structure of the source fingers 502 and drain finger 504 includesohmic metal. The source fingers 502 and drain fingers 504 may befabricated using an alloyed metal structure forming ohmic metaldeposited on a respective source finger 502 region and drain finger 504region of doped semiconductor. In various embodiments, the alloyed metalstructure includes one or more layers of Ni, Ge, Au, Cu, etc., invarious alloys and combinations of layers. The wafer may be heated sothat the metals alloy (or diffuse) into the epitaxial layer creating thelow resistance connections. Source fingers 502 and drain fingers 504 mayfunction interchangeably.

The gate fingers 506 comprise a set of layers of various combinationsand/or alloys of deposited metals (e.g., Ti, Pt, Au, Al, Ti, and/or W).The deposited metals form a Schottky contact with the surface of theepitaxial layer, creating the Schottky diode structure in the region ofepitaxial layer that comprises the gate region. The gate channel 518 mayprovide spacing for the gate fingers 506 between the source fingers 502and the drain fingers 504. In various embodiments, the length of thegate channel 518 is about 0.1, 0.25, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,or more microns. While the gate fingers 506 may not employ ohmic metalsthey are included as part of the ohmic layer 510.

FIG. 9 illustrates details of the topology of the first metal layer 520of FIG. 5A. The first metal layer includes source electrodes 522, drainelectrodes 524, and gate electrodes 526 which are disposed on the firstdielectric layer 528. Source electrodes 522, drain electrodes 524, andgate electrodes 526 are configured to carry substantially more currentwithout failing than the ohmic metal of the source fingers 502 and drainfingers 504. Further, source electrodes 522, drain electrodes 524,and/or gate electrodes 526 may be a very good heat conductor andconfigured to conduct heat substantially more efficiently than thesemiconductor layer 550 and/or the insulating substrate 560.

Referring to FIG. 5A-FIG. 9, source vias 512 disposed on the sourcefingers are connected to source electrodes 522 and are configured tocouple source current between the source fingers 502 and the sourceelectrodes 522. Drain vias 514 disposed on the drain fingers 504 areconnected to drain electrodes 524 and are configured to couple draincurrent between the drain fingers 504 and the drain electrodes 524. Gatevias 516 disposed on gate pads 508 are connected to gate electrodes 526and are configured to couple a gate signal between the gate pads 508 andthe gate electrodes 526. The source vias 512, drain vias 514, and gatevias 516 may be embedded in the first dielectric layer 528. The firstdielectric layer 528 may cover the entire surface of the ohmic layer 510of the FET device 500, which may have the effect of embedding the viasand sealing the ohmic metal and/or gate metal from the outsideenvironment and may reduce a need for an external package to protect theFET device 500. In various embodiments, source vias 512, drain vias 514,gate vias 516, source electrodes 522, drain electrodes 524, and/or gateelectrodes 526 are fabricated using Au, Cu, Al, W, Ag, and/or the like.In some embodiments, the source vias 512, the drain vias 514, and/orgate vias 516 are fabricated during the same step as the sourceelectrodes 522, drain electrodes 524, and gate electrodes 526,respectively, and may be contiguous with the respective electrodes.

As can be seen in FIG. 8, each gate finger 506 receives a gate signalfrom gate pads 508 disposed on either end of the gate finger 506. Thus,each gate finger 506 may receive a gate signal from both ends. This isdifferent from the standard FET structure as illustrated in FIG. 1 wherethe gate fingers 106 are all connected to a single large gate pad 116disposed on one end of the gate fingers 106. The gate pads 508 of FIG. 8are each configured to contact two gate fingers 506 in an alternating(meander or serpentine) pattern such that each end of each gate finger506 is connected to one gate pad 508. The meander pattern in FIG. 8 mayimprove yield by reducing lift-off problems which are characteristic ofenclosed features during fabrication. In alternative embodiments (notillustrated), each gate pad 508 may be configured to contact more thantwo of the ends of the gate fingers 506.

The serpentine structure for the gate fingers 506 and gate pads 508illustrated in FIG. 8 addresses a problem contributing to low yield dueto breaks in the gate fingers 506 discussed elsewhere herein. A majoryield driver for typical power and/or switching FET devices is breaks inthe gate fingers due to defects during fabrication. Such breaks canresult, for example, from particles on the order of a micron depositedduring the fabrication process. For a device that has been scaled downto increase gate periphery, such as illustrated in FIG. 4, one micronmay be several gate lengths (where one gate length is typically 0.25-0.5microns). If the gate signal were to be applied through the gate pad 116from only one end of the gate fingers 106 illustrated in FIG. 1, anysuch break may leave a portion of the gate finger 106 that is beyond thediscontinuity and unconnected from its voltage source (gate signal). Asa result, the portion of the gate finger 106 beyond the break would beunable to control the current flowing in that section of the channel118, thus, rendering the FET device 100 incapable of acting as a switchor power device.

However, when each gate finger 506 receives the gate signal from twoindependent points on either end as illustrated by the serpentinepattern in FIG. 8, such a break becomes a non-fatal flaw. The gatesignal can reach all portions of the gate fingers 506 on either side ofthe break. A section of one of the gate fingers 506 can becomeunconnected, and thus, uncontrolled, only if there are two breaks in thesame gate finger 506. However, the probability of two breaks in the samegate finger 506 may be a low as less than 0.04%. The impact on yield canbe illustrated in the following example calculations:

Suppose a switch and/or power FET device has 250 gate fingers each 4 mmin width, representing a total gate periphery of one meter. Furthersuppose that the probability (Y₀) of any single 1-mm segment of gatefinger not having a break is about 99.9%, which is a typical fabricationyield for such devices. Then the probability (Y_(f)) of there not beinga break in any one entire 4- mm gate finger would be about:Y _(f) =Y ₀ ⁴=99.6%

Thus, the probability (γ_(t)) of no breaks in any one of the 250 gatefingers is about:Y _(t) =Y _(f) ²⁵⁰=36.8%

As a result, the overall device yield for a FET device such illustratedin FIG. 1 where all the gate fingers receive a gate signal from one endonly, is γ_(t) or less than 40%.

Now consider the case where it takes two independent breaks in a singlegate finger to cause the device to fail, such as illustrated in FIG. 8.The probability (γ_(d)) of having less than two breaks in a single gatefinger is about:Y _(d)=1−(1−Y _(f))²=99.998%

For the overall device, the probability (Ydt) that there are no suchdouble breaks is about:Y _(dt)=(1−[1−Y _(f)]²)²⁵⁰=99.6%

Thus, the overall device yield in the case where the gate fingersreceive the gate signal independently from both ends, as illustrated inFIG. 8, is nearly 100%. In various embodiments, the length of gatefingers 506 can be 1, 0.5, 0.25, 0.15, microns or smaller. In someembodiments, the length of gate fingers 506 can be 100, 50, 25nanometers or smaller.

The structure illustrated in FIGS. 5-9 further solves the problem ofproviding the source signal to the source fingers 502 without usingconventional air bridge technology such as illustrated in FIG. 1. Thestructure illustrated in FIG. 8 also provides for high current operationwithout using the wide source and drain metal fingers (illustrated inFIG. 1 and FIG. 3) that are used to handle the large current densitiesfrom the air bridge contacts 112 and drain pad 114, respectively. Thediameter of source vias 512, drain vias 514, and gate vias 516, may be0.25, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20 microns and aretypically on the order of one to three microns in diameter. Referring toFIGS. 5-9, the source vias may provide a connection between each sourceelectrode and source finger 502 that is less than the length of thesource finger. Thus, the source finger 502 may be less than 8, 7, 6, 5,4, 3, 2, 1 or 0.5 microns in length. Multiple source vias 512 providefor distributing the source current along the source fingers 502.Current from each source via 512 may flow through a section around thesource finger 502 in a region of the source via 512 and out to a pointabout halfway to an adjacent source via 512. Thus, separate currentsegments are distributed in parallel to a region around each source via512 for reducing the current density along the source fingers 502.

Referring to FIG. 8 and FIG. 9, the source vias 512 are conductors thatare distributed along the width of the source fingers 502 to distributesource current. Source current may be partitioned into a plurality ofsource current segments for distribution through the source vias 512along the width of a source finger 502. Each source via 512 may conducta segment of the partitioned source current to a section of the sourcefinger 502 proximate the respective source via 512. Each of the sourceelectrodes 522 may distribute a segment of the source current to asection of a source finger 502. Each source electrode 522 may be inelectrical contact with at least one of a plurality of the source vias512 along a width of a source finger 502. The source electrodes 522 maybe disposed on the first dielectric layer 528 along the width of thesource fingers 502. As illustrated in FIG. 5A, the source electrodes 522may be oriented to cross the source fingers 502 at about right angles.Each source electrode 522 may be electrically coupled through at leastone of the source vias 512 embedded in the dielectric layer to a sectionof each of one or more source fingers 502. In various embodiments, apitch of the source electrodes 522 along the width of the source fingers502 is less than about 60, 50, 40, 30, 20, 10, 5, 1, 0.5, or 0.25microns.

Similarly, multiple drain vias 514 provide for distributing the draincurrent along the drain fingers 504, thus, reducing the current densityin the drain fingers 504. The drain vias 514 are conductors that aredistributed along the width of the drain fingers 504 to distribute draincurrent. Drain current may be partitioned into a plurality of draincurrent segments for distribution through the drain vias 514 along thewidth of a drain finger 504. Each drain via 514 may conduct a segment ofthe partitioned drain current to a section of the drain finger 504proximate the respective drain via 514. Each of the drain electrodes 524may distribute a segment of the drain current to a section of a drainfinger 504. Each drain electrode 524 may be in electrical contact withat least one of a plurality of the drain vias 514 along a width of adrain finger 504. The drain electrodes 524 may be disposed on the firstdielectric layer 528 along the width of the drain fingers 504. Asillustrated in FIG. 5A, the drain electrodes 524 may be oriented tocross the drain fingers 504 at about right angles. Each drain electrode524 may be electrically coupled through at least one of the drain vias514 embedded in the dielectric layer to a section of each of one or moredrain fingers 504. In various embodiments, a pitch of the drainelectrodes 524 along the width of the drain fingers 504 is less thanabout 60, 50, 40, 30, 20, 10, 5, 1, 0.5, or 0.25 microns.

The gate periphery (the product of the number of gate fingers times theaverage width of each gate finger) may be driven by the sizes of thesource fingers 102, the drain fingers 104, the channels 118, and thegate fingers 106. Smaller lengths may reduce the size of a device andlarger widths of these features may increase the gate periphery. Invarious embodiments, the gate periphery of the FET device 500 is about200, 300, 400, 500, 600, 700, 800, 900, 1,000, 1,500, 2,000 or moremicrons.

FIG. 10 illustrates details of a second metal layer 540 of FIG. 5A. Thesecond metal layer includes a source connection pad 542, a drainconnection pad 544 and a gate connection pad 546. The source connectionpad 542 is configured to provide connection for source current betweenthe FET device 500 and a circuit board and/or another device. The drainconnection pad 544 is configured to provide connection for drain currentbetween the FET device 500 and a circuit board and/or another device.The gate connection pad 546 is configured to provide connection for gatesignals from the FET device 500 to a circuit board and/or anotherdevice. In some embodiments, the circuit board may carry the sourcecurrent, drain current, and/or gate signal to other devices, e.g., in aconverter circuit. The source connection pad 542, drain connection pad544 and/or gate connection pad 546 may be connected directly to otherdevices.

Referring to FIG. 9 and FIG. 10, metal vias are disposed between thefirst metal layer 520 and second metal layer 540. The metal vias may beembedded in the second dielectric material. The metal vias, includesource vias 532, drain vias 534, and gate vias 536. The source vias 532are configured to connect the source electrodes 522 to the sourceconnection pad 542. The drain vias 534 are configured to connect thedrain electrodes 524 to the drain connection pad 544. The gate vias 536are configured to connect the gate electrodes 526 to the gate connectionpad 546.

When a wafer is singulated into individual FET devices, the sourceconnection pad 542, drain connection pad 544, and gate connection pad546 enable the die to be bonded onto a carrier substrate or packageusing standard bumping and surface mount technologies. Note that becauseof the bilateral nature of the FET structure itself, current can flowthrough the switch or power device in either direction and the sourceand drain elements and contacts are effectively interchangeable.

While FIGS. 5-10 illustrate a device including components comprisingfive source fingers 502, six drain fingers 504, ten gate fingers 506,two source electrodes 522, two drain electrodes 524, two gate electrodes526, one source connection pad 542, one drain connection pad 544, andone gate connection pad 546, more or fewer of each may be used forfabrication of the device of FIGS. 5-10. Also, more or fewer source vias512, source vias 532, drain vias 514, drain vias 534, gate vias 516,and/or gate vias 536 may be used to for fabrication of the FET device500. FIGS. 1-13 are not to scale.

FIGS. 11-13 illustrate alternative embodiments of the layout illustratedin FIG. 8, in accordance with various aspects of the technology. Thegate pads 508 are omitted for simplicity. In FIGS. 11-13, source fingers502 include source pads 602 and source lines 612. Similarly, drainfingers 504 include drain pads 604 and drain lines 614. The pads may belarger than the lines to accommodate vias. The source pads 602 arelocations for source vias 512 distributed along the width of the sourcefingers. Similarly, the drain pads are locations for drain vias 514distributed along the width of the drain pads. The sizes of the sourcepads 602 and drain pads 604 may be configured to support the source vias512 and drain vias 514, respectively. Thus, the length of the sourcefingers 502 and the drain fingers 504 may be decreased and may besmaller than the length of the source pads 602 and drain pads 604,respectively. The source pads 602 and the drain pads 604 are distributedalong the width of the source fingers 502 and drain fingers 504,respectively.

The layout 1100 of FIG. 11 illustrates an alternative embodiment of thelayout of the ohmic layer 510 illustrated in FIG. 5A and in more detailin FIG. 8, in accordance with various aspects of the invention. FIG. 11shows source line 612 and drain line 614 having a length as small as0.25-1.5 micron. Such length may be adequate for most switchapplications provided the distance along the width axis which currentmust travel on the source line 612 and drain line 614 is short enoughthat the contribution to the total resistance of the source finger 502and drain finger 504 is small. The source lines 612 may alternate withsource pads 602 to reduce current density in each source line 612.Similarly, the drain lines 614 may alternate with drain pads 604 toreduce current density in each drain line 614. The positions of thesource pads 602 may be offset relative to drain pads 604. Thus, theoverall surface area required to accommodate a given amount of gateperiphery can be further reduced. For example, the structure shown inFIG. 11 may be 33% more area efficient than the layout illustrated inFIG. 8.

The layout 1200 of FIG. 12 illustrates an alternative embodiment of thelayout of the ohmic layer 510 illustrated in FIG. 5A and in more detailin FIG. 8, in accordance with various aspects of the invention. Thelayout 1300 of FIG. 13 illustrates an alternative embodiment of thelayout of the ohmic layer 510 illustrated in FIG. 5A and in more detailin FIG. 8, in accordance with various aspects of the invention. Thelayout of FIG. 12 may be used for applications that involve switching orcontrolling relatively low currents. Referring to FIGS. 12 and 13, thesource pads 602 and drain pads 604 may be configured for supporting vias512 and 514, respectively. The source lines 612 are disposed betweenadjacent source pads 602. The drain lines 614 are disposed betweenadjacent drain pads 604. The source pads 602 and drain pads 604 may befurther separated and their size reduced by reducing the number of vias512 and 514, respectively, to as few as one, as illustrated in FIG. 12.Thus, the area used to support a given amount of gate periphery may befurther compacted. The example shown in the FIG. 12 may have a ratio ofgate periphery to overall surface area of about 0.143 μm/μm². As theseparation between source pads 602 and/or drain pads 604 becomesgreater, the ratio of gate periphery to overall surface area mayasymptotically approach about 0.167, for example where the sourcefingers 502 and drain fingers 504 have a length of about one micron andthe gate channel 518 has a length of about five microns.

While the layouts illustrated in FIGS. 11-13 illustrate alternativeembodiments of the layout of the ohmic layer 510 additional alternativelayouts employing the similar general design principles are alsopossible. In various embodiments illustrated in FIGS. 11-13, the sourcepads 602 may be separated by source lines 612 of about 1, 2, 3, 4, 5,10, 15, 20, 30, 40, 50 microns or more in width. Similarly, the drainpads 604 may be separated by drain lines 614 of about 1, 2, 3, 4, 5, 10,15, 20, 30, 40, 50 microns or more in width. In various embodimentsillustrated in FIGS. 11-13, each source pad 602 and/or drain pad mayinclude 1, 2, 3, 4, 5, 10 or more vias.

Table 1 illustrates an exemplary comparison of various parameters for alayout for a prior art power FET as illustrated in FIG. 1, andembodiments of compound semiconductor FET devices such as illustrated inFIGS. 5A, 11, 12, and 13. The column labeled “Gate Periphery” representstotal gate periphery in microns that are within an exemplary unit cell,which may be determined as the product of the number of gates and thewidth of the gates. The column labeled “Length” and “Width” representthe length and width, respectively, in microns of the unit cell. Thecolumn labeled “Ratio” represents the ratio of the total gate peripheryto the area of the unit cell (Length×Width). The units for the ratio ofthe total gate periphery to the unit cell area are microns and squaremicrons, respectively. The areas in the column labeled “Die Area”represents calculated area in square millimeters for a device having atotal gate periphery of about 1 meter (1,000 millimeters). The columnlabeled “Gross Die/Wafer” represents an estimate of the number of diethat may be fabricated on a wafer that has either a 4 inch diameter (4″column) or a 6 inch diameter (6″ column).

TABLE 1 Unit Cell Die Gross Gate Ratio Area Die/Wafer Periphery LengthWidth μm/μm² (mm²) 4″ 6″ Prior Art 200 70 100 0.029 35 224 504 Power FET(FIG. 1) FET 600 72 100 0.083 12 638 1,436 illustrated in FIG. 5A FET800 72 100 0.111 9 844 1,898 illustrated in FIG. 11 FET 600 42 100 0.1437 1,075 2,420 illustrated in FIG. 12 FET 248 36 48 0.144 7 1,180 2,530illustrated in FIG. 13

FIG. 14 illustrates a typical circuit diagram for a buck convertercircuit 1400. In the buck converter circuit 1400 there are two switchdevices known as the control (or high side) FET 1408 and the sync (orlow side) FET 1418. As can be seen in the diagram, the control FET'ssource terminal 1406 to connected directly to the sync FET's drainterminal 1412. A node 1410 between the two devices is also connected tothe converter's output through an inductor 1420 of an LC network. Notethat in this diagram the devices are shown as MOSFETs where the switchcontrol is connected to the back side of the device and the front sidegate terminal is connected directly to the source. For a compoundsemiconductor FET device the switch control is the front side gate andthere is no need to have a bias on the back side of the die.

FIG. 15 illustrates an elevation view of a prior art implementation of adevice 1500 for the buck converter circuit 1400 of FIG. 14. The device1500 uses conventional silicon MOSFET devices. The relative sizes of thecomponents of FIG. 15 are not shown to scale. In FIG. 15, the controlFET 1408 and sync FET 1418 are illustrated as fabricated using separatedie disposed on a substrate 1510, such as a printed circuit board. Thecontrol FET 1408 includes a drain 1502 and a source 1506. The sync FET1418 includes a drain 1512 and a source 1516. FIG. 15 illustrates a flowof current through the control FET 1408 and the sync FET 1418. Flow ofcurrent A-H may be represented by the bold arrows including arrowslabeled “Current A” through “Current H.” Current may be seen to flowthrough the control FET 1408 beginning at a copper strap 1508 thatextends from the metal pad on the circuit board to the top of the MOSFETdie. The current flows from the copper strap 1508 through the drain 1502(Current C) to the source 1506 (top to bottom) when a gate (notillustrated) controlled by the lead 1404 switches the FET to its ONstate. This then means that the bottom of the die of the control FET1408 must be electrically connected to the top of the die of the syncFET 1418 via a lead 1522 for Currents D and E to reach a copper strap1518. Current F flows through a copper strap 1518, which is connected tothe inductor 1420 that, in turn, is connected to the converter output.

Moreover, during each cycle, the states of the control FET 1408 and thesync FET 1418 may switch so that the control FET 1408 is in the OFFstate and the sync FET 1418 is in the ON state. When the sync FET is inthe ON state, current flows from the inductor 1420 back up copper strap1518 to the sync FET drain electrode. This flow of current may berepresented as a reverse of Current G. From the sync FET drainelectrode, current flows through the sync FET 1418 (Current H) to syncFET source 1516 which is connected to electrical ground.

In addition, since heat is generated in the bulk of the device, a heatsink (not illustrated) is normally applied to both the top and bottomsurfaces of control FET 1408 and sync FET 1418 to minimize thetemperature rise. These requirements lead to a relatively complexpackaging problem as shown in FIG. 15. Parasitic capacitance andinductance may result from the connections from the control FET source1506 to the copper strap 1518 and through the copper strap 1518 to theinductor 1420 during one part of the converter cycle, and from theinductor 1420 back through the copper strap 1518 to the sync FET drain1512 for the other part of the converter cycle. Such parasiticcapacitances and inductances may result in a decrease in switching speedof the device 1500.

A compound semiconductor FET switch fabric architecture as illustratedin FIGS. 4-13 may be adapted to eliminate this complicated packagingproblem. This may be made possible by fabricating both the control andsync switch devices on the same die, which further makes it possible tointegrate critical connections between the control FET and sync FET intothe device layout.

FIG. 16 illustrates details of a layout of an ohmic layer 1600 of FIG.5A for implementing the buck converter circuit 1400 of FIG. 14 inaccordance with embodiments of the invention. The layout of the ohmiclayer 1600 of FIG. 16 differs from the layout of FIG. 8 in that thereare two compound semiconductor devices including the control FET 1610and the sync FET 1620 that are integrated onto a single ohmic layer1600. The control FET 1610 of the ohmic layer 1600 includes sourcefingers 1606 alternating with drain fingers 1602. A serpentine gatefinger 1604 is disposed in a gate channel between each adjacent sourcefinger 1606 and drain finger 1602.

Similarly, the sync FET 1620 of the ohmic layer 1600 includes sourcefingers 1616 alternating with drain fingers 1612. A serpentine gatefinger 1614 is disposed in a gate channel between each adjacent sourcefinger 1616 and drain finger 1612 of the sync FET 1620. As in FIG. 5A,ohmic metals provide low resistance contact to the compoundsemiconductor material of the ohmic layer 1600. The structure of thesource fingers 1606 and 1616, and the drain fingers 1602 and 1612includes ohmic metal. The source fingers and drain fingers may befabricated using an alloyed metal structure forming ohmic metaldeposited on a respective source finger 1606 and 1616 region and drainfinger 1602 and 1612 region of doped semiconductor. The wafer may beheated so that the metals alloy (or diffuse) into the epitaxial layercreating the low resistance connections.

The gate fingers 1604 and 1614 may be formed as a Schottky contact asdescribed with respect to FIG. 5A. The gate channel may provide spacingfor the gate fingers 1604 and 1614 between respective the source fingersand the drain fingers. While the gate fingers 1604 and 1614 may notemploy ohmic metals they are included as part of the ohmic layer 1600.

FIG. 16 further differs from FIG. 5A in that each of the source fingers1606 in the control FET 1610 is directly connected to one of the drainfingers 1612 in the sync FET at a node 1630. As discussed elsewhereherein, the symmetrical nature of the ohmic metal structure of a FETresults in the source and drain fingers being interchangeable. Thus, thedirect connection between the ohmic metal of each source finger 1606 anddrain finger 1612 creates a continuous ohmic metal structure comprisingboth the source finger 1606 and drain finger 1612. The continuous ohmicmetal structure forms an integral and distributed connection between thetwo devices at node 1630. This may be thought of as the node 1410 in thebuck converter circuit 1400 of FIG. 14.

Further, the serpentine pattern of the gate fingers 1604 includes dualpads 1628 similar to the pads 508 of FIG. 8. Likewise, the serpentinepattern of the gate fingers 1614 includes dual pads 1638 similar to thepads 508 of FIG. 8. The second set of gate pads 1638 of the dual set ofgate pads and the small separation in between the control FET 1610 andsync FET 1620 provide only an incremental area increase for the switchfabric of a device using the layout of the ohmic layer 1600 (comprisingdevices 1610 and 1620), as compared to the area of the two switchdevices if they were fabricated separately. Thus, the manufacturing costfor the switch fabric of the device using ohmic layer 1600 is onlymarginally greater than the manufacturing cost of making the twoswitches separately and offset by the cost of connecting and mountingtwo separate devices as illustrated in FIG. 15.

The control FET 1610 of the layout of the ohmic layer 1600 furtherincludes drain vias 1622 disposed on the drain fingers 1602. These aresimilar to drain vias 514 of FIG. 8. The control FET 1610 furtherincludes source vias 1626, which are disposed on source fingers 1606.These are similar to source vias 502 of FIG. 8. Gate vias 1624 aredisposed on the dual gate pads 1628A and 1628B of the control FET 1610.These are similar to gate vias 516 of FIG. 8.

The sync FET 1620 of the layout of the ohmic layer 1600 includes drainvias 1632 disposed on the drain fingers 1612 and source vias 1636disposed on the source fingers 1616. These are similar to drain vias 514and source vias 512, respectively, of FIG. 8. Gate vias 1634 aredisposed on the dual gate pads 1638A and 1638B of the sync FET 1620.These are similar to gate vias 516 of FIG. 8.

FIG. 17 is a breakaway view illustrating details of a topology of analternate embodiment of a first metal layer 1700 in relation to theohmic layer 1600. Portions of the first metal layer 1700 are illustratedas broken away to reveal underlying structures of the ohmic layer 1600.The first metal layer 1700 may be separated from the ohmic layer 1600using a first dielectric layer 528. The first metal layer 1700 of FIG.17 differs from the first metal layer 520 of FIG. 5B and FIG. 9 in thatthe first metal layer 1700 includes source electrodes, gate electrodesand drain electrodes for two compound semiconductor FET devices, i.e.,control FET 1610 and sync FET 1620. The first metal layer 1700 includessource electrodes 1706 for the control FET and source electrodes 1716for the sync FET. These electrodes are similar to source electrodes 522of FIG. 5A. Source electrodes 1706 may be connected to source fingers1606 through source vias 1626. Source electrodes 1716 may be connectedto source fingers 1616 through source vias 1636.

The first metal layer 1700 further includes drain electrodes 1702 and1712 for control FET 1610 and sync FET 1620, respectively, similar todrain electrodes 524 of FIG. 5A. The drain electrodes 1702 may beconnected to drain fingers 1602 through vias 1622, and the drainelectrodes 1712 may be connected to drain fingers 1612 through drainvias 1632.

The first metal layer 1700 also includes dual gate electrodes 1704 anddual gate electrodes 1714 for control FET 1610 and sync FET 1620,respectively, similar to gate electrodes 526 of FIG. 5A. The dual gateelectrodes 1704 may be connected to dual gate pads 1628 through vias1624, and the dual gate electrodes 1714 may be connected to dual gatepads 1638 through vias 1634.

The drain electrodes 1702, the source electrodes 1706, and dual gateelectrodes 1704 are electrodes for the control FET 1610. The drainelectrodes 1712, source electrodes 1716 and dual gate electrodes 1714are electrodes for the sync FET 1620. Depending on parasitic resistance,drain electrodes may be omitted for the control FET 1610 because of thedirect connection between source finger 1606 and corresponding drainfinger 1612 at node 1630. Source electrodes 1716, drain electrodes 1702,drain electrodes 1712, source electrodes 1706, gate electrodes 1704, andgate electrodes 1714 are continuous but are illustrated broken acrossthe center for clarity to reveal portions of the layout of the ohmiclayer 1600. While only one drain electrode 1702 and one source electrode1706 are illustrated in FIG. 17, the control FET 1610 may include aplurality of drain and source electrodes and respective vias distributedalong the width of the drain finger 1602 and source finger 1606. Vias1622, 1624, 1626, 1632, 1634, 1636 extend through the first dielectriclayer 528 to connect the ohmic layer 1600 to the first metal layer 1700.While one drain electrode 1702 and one source electrode 1706 areillustrated in FIG. 17, more source electrodes 1706 and/or drainelectrodes 1702 may be disposed in embodiments of the first metal layer1700. While two drain electrodes 1712 and two source electrodes 1716 areillustrated in FIG. 17, more or fewer drain electrodes 1712 and/orsource electrodes 1716 may be disposed in embodiments of the first metallayer 1700.

FIG. 18 illustrates a top plan view of the first metal layer 1700 ofFIG. 17. The ohmic layer 1600 is omitted from FIG. 18 for clarity. Thefirst metal layer 1700 includes drain vias 1802 disposed on the topsurface of drain electrodes 1702, source vias 1806 disposed on the topsurface of source electrodes 1706, and gate vias 1804 disposed on thetop surface gate electrodes 1704 of the control FET 1610. Vias 1802,1804, and 1806, are omitted from FIG. 17 for clarity. The first metallayer 1700 further includes drain vias 1812 disposed on the top surfaceof drain electrodes 1712, gate vias 1814 disposed on the top surface ofgate electrodes 1714, and source vias 1816 disposed on the top surfaceof source electrodes 1716, of the sync FET 1620. Vias 1812, 1814, and1816 are omitted from FIG. 17 for clarity.

FIG. 19 illustrates a top plan view of the second metal layer 1900. Thefirst metal layer 1700 is omitted from FIG. 19 for clarity. FIG. 20illustrates the second metal layer 1900 in relation to the first metallayer 1700. The second metal layer 1900 may be separated from the firstmetal layer 1700 using a second dielectric layer 538. The second metallayer 1900 of FIGS. 19 and 20 differs from the second metal layer 540 ofFIG. 10 in that the second metal layer 1900 includes source, gate anddrain electrodes for two compound semiconductor FET devices, i.e.,control FET 1610 and sync FET 1620.

The electrodes of the first metal layer 1700 are illustrated in FIG. 20in dotted line to indicate that they are below the second metal layer1900. Vias 1802, 1804, 1806, 1812, 1814, and 1816 are also illustratedin dotted line in FIGS. 19 and 20 to indicate that they are disposed inthe second dielectric layer 538 between the first metal layer 1700 andsecond metal layer 1900. Vias 1802, 1804, 1806, 1812, 1814, 1816 extendthrough the second dielectric layer 538 to connect the first metal layer1700 to the second metal layer 1900.

The second metal layer includes a drain connection pad 1902, a sourceconnection pad 1906, and a gate connection pad 1904 for the control FET1610. The second metal layer further includes a drain connection pad1912, a gate connection pad 1914 and a source connection pad 1916 forthe sync FET 1620.

The drain connection pad 1902 is connected to the drain electrodes 1702using the drain vias 1802. The drain vias 1802 are illustrated in dottedline to indicate that they are between the drain connection pad 1902 ofthe second metal layer and the drain electrodes 1702 of the first metallayer.

The source connection pad 1906 is connected to the source electrodes1706 using the source vias 1806. The source vias 1806 are illustrated indotted line to indicate that they are between the source connection pad1906 of the second metal layer and the source electrodes 1706 of thefirst metal layer.

The gate connection pad 1904 is connected to the gate electrodes 1704using the gate vias 1804. The gate vias 1804 are illustrated in dottedline to indicate that they are between the gate connection pad 1904 ofthe second metal layer and the gate electrodes 1704 of the first metallayer.

The drain connection pad 1912 is connected to the drain electrodes 1712using the drain vias 1812. The drain vias 1812 are illustrated in dottedline to indicate that they are between the drain connection pad 1912 ofthe second metal layer and the drain electrodes 1712 of the first metallayer.

The gate connection pad 1914 is connected to the gate electrodes 1714using the gate vias 1814. The gate vias 1814 are illustrated in dottedline to indicate that they are between the gate connection pad 1914 ofthe second metal layer and the gate electrodes 1714 of the first metallayer.

The source connection pad 1916 is connected to the source electrodes1716 using the source vias 1816. The source vias 1816 are illustrated indotted line to indicate that they are between the source connection pad1916 of the second metal layer and the drain electrodes 1712 of thefirst metal layer. An example of a monolithic dual device for a circuitincluding a control FET and sync FET for a buck converter circuit ispresented. However, other circuits containing two or more FET devicesmay be fabricated using the dual device layout, via design, integralsource-drain finger, and serpentine gates technologies disclosed. Forexample, if two separate switch devices were fabricated on the same diefor independent operation and not specifically intended to work togetheras a control/sync pair, then the source fingers of the control FET maynot be connected to the drain fingers of the sync FET in the ohmiclayer. Further, if two or more completely independent switches arefabricated on the same die, then there may not be an interconnection offingers from one device to the other in the ohmic metal layer. In suchcase, there would be no node 1630 (as illustrated in FIGS. 16 and 17)connecting the control source finger 1606 to the sync drain finger 1612.

However, in some embodiments, the switch devices may operate in parallel(rather than in series as illustrated in the drawings. In such a casethe respective top level pads may be connected together in the secondmetal layer, e.g., as illustrated in FIG. 21.

FIG. 21 illustrates a top plan view of an alternative embodiment of asecond metal layer 2100. The second metal layer 2100 of FIG. 21 differsfrom the second metal layer 1900 of FIG. 19 in that the sourceconnection pad 1906 (control FET) and the drain connection pad 1912(sync FET) form a single electronically continuous connector pad 2102.Thus, the source fingers 1606 of the control FET, which form acontinuous finger with the drain fingers 1602 of the sync FET may beelectronically coupled through a single connector pad 2102 to externalcomponents. This may reduce the number of contacts for using the syncFET and control FET from six to five contacts, i.e., control gateconnection pad 1904, the sync gate connection pad 1914, the controldrain connection pad 1902, the sync source connection pad 1916 andswitch node connection pad 2102.

FIG. 22 is a block diagram of a side elevation illustrating layers of aFET device 2200 of FIGS. 16-21. As discussed elsewhere herein, the vias1622-1636 (not visible in FIG. 22) extend through the first dielectriclayer 528 to connect features in the ohmic layer 1600 to electrodes inthe first metal layer 1700. Similarly, vias 1802-1816 (not visible inFIG. 22) extend through the second dielectric layer 538 to connectelectrodes in the first metal layer 1700 to connection pads in thesecond metal layer 1800.

As used in this specification, the terms “include,” “including,” “forexample,” “exemplary,” “e.g.,” and variations thereof, are not intendedto be terms of limitation, but rather are intended to be followed by thewords “without limitation” or by words with a similar meaning.Definitions in this specification, and all headers, titles andsubtitles, are intended to be descriptive and illustrative with the goalof facilitating comprehension, but are not intended to be limiting withrespect to the scope of the inventions as recited in the claims. Eachsuch definition is intended to also capture additional equivalent items,technologies or terms that would be known or would become known to aperson having ordinary skill in this art as equivalent or otherwiseinterchangeable with the respective item, technology or term so defined.Unless otherwise required by the context, the verb “may” indicates apossibility that the respective action, step or implementation may beperformed or achieved, but is not intended to establish a requirementthat such action, step or implementation must be performed or mustoccur, or that the respective action, step or implementation must beperformed or achieved in the exact manner described.

The above description is illustrative and not restrictive. This patentdescribes in detail various embodiments and implementations of thepresent invention, and the present invention is open to additionalembodiments and implementations, further modifications, and alternativeconstructions. There is no intention in this patent to limit theinvention to the particular embodiments and implementations disclosed;on the contrary, this patent is intended to cover all modifications,equivalents and alternative embodiments and implementations that fallwithin the scope of the claims. Moreover, embodiments illustrated in thefigures may be used in various combinations. Any limitations of theinvention should, therefore, be determined not with reference to theabove description, but instead should be determined with reference tothe appended claims along with their full scope of equivalents.

What is claimed is:
 1. A method for switching current using a controlFET and a sync FET, the method comprising: partitioning a current into aplurality of current segments; distributing the plurality of currentsegments to sections of a control drain element of a control FET, thecurrent segments distributed through a plurality of vias distributedalong a surface of the control drain element; distributing the pluralityof current segments to sections of a control source element of a controlFET, the current segments distributed through a plurality of viasdistributed along a surface of the control source element; coupling acontrol gate signal to a first and second end of a control gate fingerdisposed between the control drain element and a control source elementof the control FET; switching the plurality of current segments from thecontrol drain element to the control source element using the controlgate finger; conducting the switched current segments along a continuousohmic metal from the control source element of the control FET to a syncdrain element of a sync FET; and extracting the plurality of currentsegments from sections along the sync drain element through a pluralityof vias distributed along a surface of the sync drain element.
 2. Themethod of claim 1, further comprising: coupling a sync gate signal toboth a first and a second end of a sync gate finger disposed between thesync drain element and a sync source element of the sync FET switchingthe plurality of current segments from the sync drain element to thesync source element using the sync gate finger; and extracting theswitched current segments from sections along the sync source elementthrough a plurality of vias distributed along a surface of the syncsource element.
 3. The method of claim 1, further comprising removingheat from the control FET and the sync FET through a control drainelectrode, a sync drain electrode, and a sync source electrodeelectrically coupled, respectively, to the control drain element, thesync source element, and the control drain element.
 4. The method ofclaim 1, wherein the control FET and the sync FET are fabricated on apiece of gallium arsenide or gallium nitride.